1. Field of the Invention
The present invention is generally concerned with pulley assemblies of the kind comprising a rotatable shaft, two flanges keyed to said shaft so as to rotate therewith, and a variable transmission device disposed axially between and in contact with said flanges and able to move radially relative thereto, one of said flanges, hereinafter referred to as the mobile flange, being mounted so as to move axially relative to said shaft between two extreme positions, namely a retracted position and an advanced position.
2. Description of the prior art
This type of pulley is usually employed, for example, in a variable transmission drive system.
The variable transmission device is then in practice a belt passing in an endless loop around two pulleys, the shaft of one of which forms the driving input member of the assembly whereas the shaft of the other forms the driven output member.
Variable transmission drive systems comprising such pulley assemblies are intended in particular for equipping automobile vehicles, for the purpose of driving ancillary devices such as the alternator, water pump, air conditioning plant and the like.
They provide, to at least a certain extent, for regulating the speed at which these ancillary devices are driven so as to be independent of the rotation speed of the motor: on starting the motor the mobile flange of the driving pulley is in the advanced position whereas that of the driven pulley is in the retracted position. The pulleys then provide a first and relatively high transmission ratio, of the order of 1.3:1, for example. When the motor has run up to its normal rotation speed the reverse arrangement is established and the pulleys then provide a relatively low transmission ratio, of the order of 0.6:1, for example.
The present invention is more particularly, but not exclusively, concerned with the case in which the changeover from one transmission ratio to the other is effected automatically, in response to centrifugal force.
For example, the mobile flange of the pulleys may be urged towards the other flange by diaphragm spring type elastic means, that is to say by elastic means comprising a circumferentially continuous peripheral part forming a Belleville washer and bearing on the mobile flange, and a central part subdivided into radial fingers bearing on a support member fixed in the axial direction relative to the shaft of the pulley assembly, these elastic means being acted on by flyweights which are responsive to centrifugal force and modify the loading applied in accordance with the speed of rotation.
There are then three consecutive phases of operation to be considered.
On starting, the flyweights are initially without significant effect. During this first phase of operation the variable transmission drive system provides a fixed transmission ratio and this is the highest transmission ratio.
Then, during a second phase of operation which begins when the flyweights begin to have some effect, it provides a variable transmission ratio which progressively decreases.
Finally, in a third phase of operation, the flyweights are rendered inoperative and the variable transmission drive system again provides a fixed transmission ratio, and this is the lowest transmission ratio.
Although such an arrangement has given and can continue to give satisfaction, it has the disadvantage that it is not necessarily adapted to present requirements.
It systematically imposes a high transmission ratio on the variable transmission drive system when the motor driving it is idling.
It is not necessarily certain that the ancillary devices driven need to be driven at full speed at this time, however.
In the case of the alternator, for example, such "forced" operation is in practice beneficial only when the load on the accumular battery is high for other reasons, for example due to the simultaneous use of main beam or dipped headlights, a demister and/or an electric window actuator.
It is not necessary in order to maintain the accumulator battery charged under normal conditions, however.
Apart from the excessive and unnecessary fatigue of the ancillary devices which results, the "forced" driving of these devices due to a high transmission ratio of the variable transmission drive system controlling them gives rise to a wasteful consumption of energy.
A general objective of the present invention is to provide a device enabling this disadvantage to be overcome and thus enabling energy to be saved.